1. Field of the Invention
The present invention relates in general to the art of diffusion bonding and, more particularly, to diffusion bonding of the superalloys utilizing a transient liquid phase in the bonding process.
2. Description of the Prior Art
Transient liquid phase diffusion bonding has been shown to be a very useful method for producing high quality diffusion bonds in the high temperature superalloys, such as those used in the manufacture of gas turbine engine hardware. Such bonding is described in detail in the U.S. Pat. No. 3,678,570 to D. F. Paulonis et al, of common assignee herewith, and whose teachings are incorporated herein by reference.
The superalloys are recognized as those alloys usually having basis in nickel, cobalt or iron, or some combination thereof and exhibiting good high temperature strength and oxidation resistance in environments such as gas turbine engines. Usually, these alloys also contain substantial quantities of chromium and also a limited amount of other elements such as aluminum, titanium and the refractory metals.
It is frequently desirable to make certain gas turbine engine components by joining easily fabricable segments together into the desired configurations. However, the limited weldability of many of the superalloys has severely limited the applicability of fusion welding techniques in the production of structural hardware. Further, many components, because of their design, are simply not adapted to the utilization of fusion welding. Brazing, while offering a number of advantages over fusion welding, has very limited application because of the penalties such as relatively low strengths and low melting points associated with the typical brazed joints.
The relative simplicity and reproducibility of the transient liquid phase diffusion bonding technique in the production of high quality bonds in sensitive hardware has led to substantial usage thereof. This is particularly true in the gas turbine engine industry, although the invention described herein is obviously not limited thereto.
One key element in the transient liquid phase diffusion bonding technique is the provision, between the surfaces to be joined, of a thin alloy interlayer. The composition of the interlayer preferably and generally should be tailored to the alloys being joined, particularly with respect to the inclusion therein of those elements whose presence is required in the finished bond area and whose solid state diffusion rates are low. It is also desirable to exclude from the interlayer alloy those elements which may adversely affect the bonding process or the quality of the finished joint. In all of the interlayer compositions a high boron content is utilized.
Methods for providing the interlayer alloy between the faying surfaces are described in the Paulonis et al patents, U.S. Pat. Nos. 3,753,794 and 4,005,988, both of which share a common assignee herewith. In one case, the interlayer alloy is in the form of a thin, ductile foil while in the other it is in the form of a coating deposited on one or more of the faying surfaces, the coating containing as its basis metal the base metal of the superalloys and a quantity of boron as a melting point depressant. Although these techniques have proved highly successful, the search for still less expensive and time consuming methods for providing the interlayer alloy has continued.